1. Field of the Invention
The present invention relates to the design of reticles used in photolithographic processing of substrates such as semiconductor wafers. More specifically, the present invention relates to reticles having a layer of a polarized material and methods of exposing the reticle using polarized light to produce a high resolution image on a photoresist.
2. State of the Art
Semiconductor devices including integrated circuitry, such as memory dice, are mass produced by fabricating hundreds or even billions of circuit patterns on a single semiconductor wafer or other bulk semiconductor substrate using photolithography in combination with various other processes. In order to increase the number of memory cells on semiconductor memory devices for a given surface area, it is important to accurately control the optical resolution of the images produced during photolithography. These images are used to define structural features on a semiconductor substrate to fabricate the integrated circuitry of such semiconductor memory devices.
Photolithography is a process in which a pattern is delineated in a layer of material, such as a photoresist, sensitive to photons, electrons, or ions. In photolithography, an object containing a pattern (e.g., reticle or mask) is exposed to incident light. The image from the reticle or mask is projected onto a photoresist that covers a semiconductor wafer or other substrate. The photolithographic process typically involves exposing and developing the photoresist multiple times with deposition, etch, and/or implant steps in between. At a given step, the photoresist is selectively exposed to photons, electrons, or ions, then developed to remove one of either the exposed or unexposed portions of photoresist, depending on whether a positive or negative photoresist is employed. A complex device typically requires multiple exposure and development steps.
There are three predominant conventional photolithography methods of optically transferring a pattern on a reticle or mask to a photoresist that is coated on a substrate. These methods are contact printing, proximity printing, and projection printing. Currently, projection printing is the most frequently used type of photolithography system. Referring to FIG. 1, a conventional photolithography system used in projection printing is shown. Photolithography system 100 includes illumination controller 102 operably coupled to illumination source 104 for producing light. Illumination source 104 typically includes a mirror, a lamp, a laser, a light filter, and/or a condenser lens system. In the exposing system shown in FIG. 1, illumination source 104 irradiates reticle 106 having the desired pattern to be projected onto photoresist 110. Projection lens 108, which may include a complex set of lenses and/or mirrors, focuses the image from reticle 106 onto photoresist 110. Photoresist 110 is developed and substrate 112 is subsequently processed as by etching to form the desired structures and photoresist 110 is then removed.
A major problem in the manufacture of memory dice and other semiconductor devices using photolithography is that the periphery region and the array region of the reticle have their largest process windows under different illumination conditions. This problem is particularly exacerbated when the feature sizes to be formed on the photoresist are small, such as around on the order of one-half of the wavelength of the light source used, or less. A typical reticle pattern of reticle 200 comprises an array region 204 and a peripheral region 202 as shown in FIG. 2A. FIG. 2B illustrates a view of a single array region 204 surrounded by the peripheral region 202. Again referring to FIGS. 2A and 2B, the array region 204 and the peripheral region 202 of reticle 200 have different patterns. For instance, the array region 204 may contain a periodic pattern with particular dimensions and the peripheral region 202 may contain a different pattern having a smaller or larger dimension, possibly a different periodic pattern, or even a nonperiodic pattern.
In conventional photolithography, the peripheral pattern and the array pattern of the reticle are exposed to the illumination source at the same time. However, the optimal illumination conditions for the array region and the peripheral region are not identical. The term “illumination condition” as used herein should be understood to include the distribution of angles of light used to irradiate the reticle and the total intensities of the light in those angles. A relatively tightly spaced pattern characteristic of the array region typically requires illumination by a circular annulus of light at a fairly steep incident angle. A relatively sparse pattern characteristic of the peripheral region typically has its optimal illumination conditions when using a single plane wave of incident light. Thus, each region of the reticle has particular illumination conditions such as depth of focus, dose and angle of incident light, among others, which have different optimal values for the array and the peripheral region. Therefore, if the illumination conditions are optimized for the array region, the illumination conditions for the peripheral region are suboptimal and vice versa. U.S. Pat. No. 5,245,470 to Keum attempts to overcome some of the problems with producing patterns using photolithography.
A possible solution to this problem is to use more than one reticle, as is commonly known in the art, and sometimes used in the fabrication of semiconductor devices. However, the use of dual reticles suffers from two main deficiencies. First, it adds the additional cost of manufacturing or buying a second reticle. Second, the use of more than one reticle decreases the process throughput by requiring the changing out of the first reticle for the second reticle and often times necessitating recalibration of the photolithography system. Third, the use of more than one reticle also causes problems with overlay errors between the two reticles.
Accordingly, in order to improve the quality of patterns transferred to photoresists using photolithography, a need exists for a single reticle photolithography system suitable for exposing regions of the reticle having distinct patterns under different illumination conditions optimal for each respective region.